Precision Time Protocol (PTP) IEEESTD 1588 is one of the important technologies in the field of time and frequency synchronization control. During practical applications, link asymmetry always is a difficult point troubling synchronization performance.
So called link asymmetry refers to that the downlink transmission speed is inconsistent with the uplink transmission speed in a network between a master clock device and a slave clock device (which is also called a slave device in this disclosure), which leads to a deviation between the time calculated by the slave clock device and the time calculated by the master clock device.
Static deviation can be measured in a steady state of the network and can be corrected by the setting of an asymmetric correction value on network devices. Dynamic deviation includes two conditions. One condition might be approximate to a random error; this type of dynamic deviation with high randomness generally long lasts in a certain frequency range, and can be eliminated by a specific filter designed on a device. The other condition is impacted by network transmission factors, and cannot be eliminated by a common filtering method.
Generally, for the asymmetry in a network, besides the correction of static deviation, the link delay change caused by forward delay in a normal link is eliminated by a transparent transmission function of clock nodes. The transparent transmission function puts the delay of the current level of transmission into a protocol message, the delay continues to be accumulated and transmitted by subsequent nodes; when the slave clock node receives the protocol packet, this part of delay is removed to greatly eliminate the impact from the link delay change.
However, if there is one or more intermediate devices on the link not supporting the transparent transmission function, for example, when the transmission is conducted via a third-party network, since the link asymmetric change caused by transmission delay on these network nodes is easy to reach a big value, a common slave clock device can not handle this condition, which directly blocks the application of 1588 clock services in existing networks.
In view of the above problem, no solution has been put forward so far.